In general, a construction machine such as an excavator drives a plurality of working units such as a boom, an arm and a bucket by using a working fluid discharged from a variable capacity hydraulic pump directly connected to an engine.
A discharge flow rate of the hydraulic pump is controlled by various parameters so as to satisfy various conditions such as work efficiency and fuel efficiency.
In more detail, a control method of a hydraulic pump includes a working flow rate control (flow control) for controlling a discharge flow rate according to a manipulation signal input from a manipulation part, a constant horse power control for controlling a discharge flow rate of the hydraulic pump according to a discharge pressure of the hydraulic pump such that a required horse power of the hydraulic pump remains constant, and a horse power control (power shift control) for controlling a discharge flow rate of the hydraulic pump according to a load condition of an engine.
In order to perform the above-mentioned control method, the hydraulic pump is provided with a regulator, and the regulator includes a working flow rate regulating part for controlling working flow rate, a constant horse power regulating part for the constant horse power control, and a horse power regulating part for the horse power control (power shift control). The working flow rate regulating part receives a negative control pressure which is center-bypassed, a pilot pressure of the manipulation part or a load sensing pressure of each actuator and controls a discharge flow rate of the hydraulic pump. The constant horse power regulating part receives a discharge pressure (load pressure) of the hydraulic pump and controls a discharge flow rate of the hydraulic pump according to a set constant horse power line diagram. Finally, the horse power regulating part controls a discharge flow amount of the hydraulic pump according to a target engine RPM set by a dial gauge of the engine according to a load of the engine calculated from the current engine RPM.
As illustrated in FIG. 1, in the above-mentioned power control apparatus, if a manipulation of the manipulation part abruptly increases, a manipulation signal is input to the working flow rate control unit, abruptly increasing a flow rate of the hydraulic pump, and accordingly, a discharge pressure of the hydraulic pump abruptly increases, causing a required horse power of the hydraulic pump to also abruptly increase. Then, as the abruptly increased discharge pressure of the hydraulic pump is input to the constant horse power regulating part, a discharge flow rate of the hydraulic pump starts to decrease.
However, a flow rate of the hydraulic pump is reduced by the constant horse power regulating part after a predetermined time from a time point where a discharge pressure of the hydraulic pump due to a response delay time of the constant horse power regulating part. The discharge pressure of the hydraulic pump continuously increases for a time period when the constant horse power control point is delayed, generating a hydraulic impact. A section where a required horse power of the hydraulic pump abruptly increases like the section A of FIG. 1 is generated by the hydraulic impact.
In this way, as an abrupt increase of a required horse power of the hydraulic pump acts as a high load to the engine, an RPM of the engine abruptly decreases below a set target RPM. If an engine RPM is abruptly lowered in this way, exhaust fumes increase and vibrations become severe as well. In particular, in a section (turbo charger time lack section) where a drive of a turbocharger reaches a normal state as in section B of FIG. 1, an output increase rate of the engine becomes lower, further lowering the above-mentioned engine RPM and further deteriorating exhaust fumes and vibrations.
Meanwhile, if an RPM of the engine is abruptly lowered from the target RPM, the horse power regulating part lowers a driving power of the hydraulic pump from a maximum horse power (200 mA) to a minimum horse power (600 mA) to increase an RAM of the engine. Accordingly, a flow rate of a working fluid discharged from the hydraulic pump becomes lower, causing a working efficiency of the construction machine to be lowered.
FIG. 2 is a constant horse power line diagram schematically illustrating the above-mentioned process. Referring to FIG. 2, it can be seen that after a discharge pressure of the hydraulic pump abruptly increases, the flow rate and pressure returns to a constant horse power line diagram again as in line diagram C.
In summary of the problems of the above-mentioned power control apparatus according to the related art, a hydraulic impact by which a required horse power of the hydraulic pump is abruptly increased is generated due to a time delay of a constant horse power control point by the constant horse power regulating part. Accordingly, an RPM of the engine abruptly decreases, causing severe exhaust fumes and vibrations. Further, a required horse power of the hydraulic pump is abruptly lowered in a process where the horse power regulating part drives the hydraulic pump at a minimum horse power to recover an RPM of the engine to a target RPM, causing a working efficiency of the construction machine to be lowered.
In describing a horse power control of the engine in more detail, if an engine RPM is lower than a target RPM, the controller outputs a control signal to the horse power regulating part to reduce a flow rate of the hydraulic pump so that the engine RPM returns to the target RPM. Further, if a discharge flow rate of the hydraulic pump is controlled to become smaller so that the RPM of the engine becomes higher than the target RPM, a control signal is output to the horse power regulating part again to increase a flow rate of the hydraulic pump. In this way, the RPM of the engine is negatively controlled by a load of the hydraulic pump, and if an engine load ratio (a load torque of the engine to a maximum torque of the engine) becomes higher, the RPM of the engine approaches the target RPM, and if the engine load ratio becomes lower, the RPM of the engine becomes higher than the target RPM. Accordingly, even when the load transferred from the hydraulic pump to the engine is low, the engine maintains a high RPM, causing much energy loss.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.